In 2014, the Ministry of Environmental Protection and the Ministry of Land and Resources released the “Bulletin on the Survey of Soil Pollution in China”, which indicated that the over-standard rates of five divalent cationic heavy metals such as cadmium, copper, lead, zinc and nickel in soil were 7.0%, 2.1%, 1.5%, 0.9% and 4.8%, respectively; the over-standard rate of arable land was 19.4%, which included slight pollution 13.7%, light pollution 2.8%, moderate pollution 1.8% and heavy pollution 1.1%, with cadmium, nickel, copper and lead as the main pollutants. Pollution abatement of cadmium and other heavy metals in soil is a major environmental issue facing China, especially pollution abatement of cadmium in rice field soil.
In our country, with a large area of rice field soil polluted by cadmium, it is urgent to develop a technical method that can be popularized and applied in a large area with low cost and no retarding of the farming operations, so as to reduce the content of cadmium in rice. In fact, the behavior of heavy metals such as cadmium in rice field soil is a surface process of water-soil-gas-biology interaction, which may be affected by the biogeochemical behavior of many elements. Among these elements, element sulfur is the most abundant active element in rice field soil inputted by human, and iron is the most active metal element with high abundance in rice field soil. Microorganisms are the main driving force for the morphological transformation of sulfur and iron in rice field soil and thus for the morphological changes of heavy metals. Biogeochemical cycling of sulfur and iron in rice field soil will affect the morphology and bioavailability of the heavy metal cadmium. By understanding the basic scientific problems of biogeochemistry in the cycle of the elements cadmium, sulfur and iron and developing products with application value, the mobility and bioavailability of cadmium in rice field soil can be reduced, so can be the absorption of cadmium in rice, thereby increasing the safety of rice in view of cadmium. This is a viable technical idea and also the only way for controlling cadmium pollution in large area of rice field soil in China.
Sulfur significantly affects the activity and bioavailability of the heavy metal cadmium. Sulfur, as an important active element in soil, has a very active geochemical process. Sulfur in soil has different valence states from −2 to +6, mainly including S2−, S0, SO42−, S2O32−, S4O62− and other ion forms. SO42− entering the soil is quickly reduced to S2− under anaerobic conditions, which can form sulfides with metal ions to stabilize heavy metals. However, when sulfur is oxidized in soil to form SO42−, a large amount of H is produced, resulting in the activation of heavy metals (CHEN Huai-man et al., 2002). Therefore, the oxidation and reduction of sulfur in soil is the key mechanism to control the dissolution and precipitation of heavy metals in soil. Soil microorganisms play a central role in the sources and sinks of sulphate, and the interaction between various microorganisms is an important regulator of the content of sulfate in soil (Wainwright, 1984).
Recently Muehe et al. isolated Geobacter sp. strain Cd1, which could reduce and dissolve cadmium-containing iron oxide minerals, with the released Cd fixed by secondary iron oxide minerals (Muehe et al., 2013). It was indicated by this study that during the reduction of iron, the re-fixation of Cd would also be promoted due to the recrystallization of iron oxide minerals. Cooper et al. found that, in this process the free divalent heavy metals (Cd, Co, Mn, Ni, Pb and Zn) in soil could be fixed in the secondary mineral structure during the recrystallization of iron oxide to achieve the structured fixed detoxification of heavy metals (Cooper et al., 2006).
According to the above principle analysis, activation of sulfur-reducing bacteria and other microorganisms in soil can promote reduction of sulfur and iron in rice field soil, and thus get cadmium fixed. The activity of sulfur-reducing bacteria and other microorganisms in soil is related to electron donors, electron shuttles and so on in soil, and can be effectively activated by electron donors and electron shuttles added to rice field soil.
Electron donors refer to low-molecular-weight organic carbon materials required for the growth of microorganisms such as sulfur-reducing bacteria, and electron shuttles refer to carriers transporting electrons between microorganisms and minerals. Electron shuttles acquire electrons from the extracellular respiration microorganisms and are reduced, and then deliver the electrons to electron acceptors (minerals such as iron oxide) and are meanwhile oxidized, with the structure of the shuttles in this process relatively stable and not consumed. Most microorganisms that drive the reduction of iron and sulfur have the function of extracellular respiration, and thus electron shuttles can activate iron-reducing and sulfur-reducing microorganisms and meanwhile accelerate the reduction of iron and sulfur in rice field soil.
Electron shuttles include micromolecular benzoquinone-based humus having the quinonyl structure, macromolecular humus and solid humus biochar. The micromolecular benzoquinone-based humus AQDS (9,10-anthraquinone-2,6-disulfonic acid) has a standard redox potential of −0.184 V, and several other kinds of quinonyl-containing humus have a standard redox potential of about −0.5 to −0.003 V. These standard redox potential values are lower than the standard redox potential values for the reduction of nitrate, sulfate and iron. In most cases, humus and like-humus can obtain electrons earlier than nitrate, sulfate and iron oxide, and deliver the electrons they have obtained to other electron acceptors including nitrate, sulfate and iron oxide. Therefore, humus and like-humus act as electron shuttles in the biogeochemical cycle of elements. Humus, as an important part of organic matter in soil, can be divided into three relatively homogeneous components according to the solubility of humus in aqueous solution: (1) fulvic acid (FA), soluble in acid and alkali; (2) humic acid (HA), only soluble in alkali solution, and precipitated after the acidification of alkali extract; and (3) humin (HM), insoluble in acid or alkali. Humus contains in the molecular structure a large number of active functional groups, such as carboxyl, alcoholic hydroxyl and phenolic hydroxyl, and thus has a high geochemical activity and can have complexation with ions of Cu, Cd, Pd, Zn and Hg and other toxic metal ions that enter the environment. Most importantly, the quinonyl group of humus determines that the humus is a class of natural organic matter with redox activity. The solid humus biochar contains in the surface a large number of phenolic, quinonyl and aromatic groups, which are all important reaction sites involved in electron transfer. Quinonyl is the most important active site for electron transfer contributing about 70%, with the contribution of the non-quinonyl site to electron transfer about 30%.
Based on the above analysis, the electron shuttles above have the function of activating microorganisms such as sulfur-reducing bacteria and iron-reducing bacteria. By utilizing these substances to activate the activity of microorganisms such as sulfur-reducing bacteria and utilizing the coupled cadmium of the cycle of the elements sulfur and iron to fix the deactivation relationship, the technical methods and new products for reducing the mobility of cadmium in rice field soil are developed. From the biogeochemical point of view of the elemental cycle, the activity of functional microorganisms in rice field soil related to cadmium deactivation is regulated, and the mobility of cadmium in rice field soil is reduced, thus achieving the goal of abating cadmium pollution in rice field soil. These are of great significance for the abatement of heavy metal pollution in large-area farmland in our country.